I’m glad you mentioned wiki posts, it got me thinking about that feature again. 
I have set up a few to try out the feature, and proposed something similar here.
While I agree, wiki posts are still posts in a forum, not pages/articles in a wiki. There are many differences to consider, not just the collaborative editing feature.
For example, a readability issue is evident in the first post of that thread: GOSH 2022 SESSIONS Documentation - THURSDAY Block # 3 (01:00 pm). Longer posts with more varied content will also be harder to edit here.
My main argument against the wiki is splitting content and participation into different websites. A massive amount work went into putting the Roadmap into GitLab issues, and this ended with only 2-4 users participating. Using Discourse wiki posts would not generate this problem.
I also imagine that re-purposing Discourse as a wiki could work. But I would be careful with planning how, and consider the amount of work that it would actually take.
While I agree with this idea, I believe that this matter it is both about motivation and tools.
I will make a few comments on this at the next Roadmap Issues meeting!
PS: I just found an example from OSMC.tv on how Discourse wiki-posts can be set up to auto-publish as articles on a static page. An “edit”/“discuss” button could be added to each one, pointing to the forum.